Fax over IP: getting a fax machine, router and VoIP line to work together
A configuration guide: keeping a fax machine in service when the line comes in over fibre, cable or an IP PBX.
Behind a router or any VoIP line, a fax machine depends on one condition: that its call undergoes no compression or delay along the way. Two methods exist — G.711 passthrough, which handles the signal the same way as ordinary voice, and T.38, which converts it into data. Set the machine to 9,600 bps at most, turn off ECM on G.711, then confirm with multi-page tests.
The key points
The two modems talk to each other from start to finish: compression, packet loss or jitter — anything that distorts the sound compromises the ongoing exchange or the page in transmission.
T.38 is built for IP networks; G.711 passthrough can also work, over a clean link with no transcoding.
Baseline machine settings: V.29, i.e. 9,600 bps; V.34 (Super G3) switched off; ECM disabled if the line is not on T.38.
A single test page proves nothing: send a multi-page document and try it at different times of day.
Fax over IP failure: the causes
In the copper days, the fax machine enjoyed a permanent circuit with stable delay. Behind a router, the signal gets digitised, chopped into 10 to 30 ms packets, and routed through a shared data network. Yet T.30 expects specific tones at specific moments: it was never designed for that world.
Three phenomena account for most failures. Codecs built for speech (G.729, Opus, AMR) discard precisely the details the modem relies on to carry its data. Jitter — packets arriving irregularly — forces the router to stretch or chop the stream. And 20 ms lost at the wrong moment is enough to destroy a scan line.
With G.711, ECM (Error Correction Mode) frequently works against you: every damaged block is requested again, retransmissions pile up, the sequence eventually runs past its timeouts, and the machine hangs up. With ECM switched off, a loss produces at worst a white line, which is still readable.
G.711 passthrough or T.38: the showdown
The internet box or gateway determines the method when the call is established.
| Criterion | T.38 (fax relay) | G.711 in passthrough |
|---|---|---|
| Operation | The signal is demodulated as soon as it enters the IP network, carried as data (UDPTL), then remodulated on exit. | The modem noise travels raw, like an uncompressed conversation. |
| Lost packets | Reliable: every packet carries copies of earlier data (redundancy). | Poorly tolerated: a loss erases a line or cuts off the page. |
| Jitter (delay variation) | Limited impact: the modem timing is reconstructed locally. | Significant impact; a fixed-size jitter buffer helps a great deal. |
| Bandwidth | Low; it rises with the redundancy used. | Around 64 kbit/s in each direction for the audio; with IP headers, allow for 80 to 90 kbit/s. |
| Requirements | On the hardware side: the box or ATA, carrier, and remote gateway must all support the switchover to T.38 via a SIP re-INVITE. | G.711 throughout the chain (A-law in Europe), no transcoding. |
| Speed used | 14,400 bit/s (V.17) works most of the time; V.34 should still be avoided. | 9,600 bit/s (V.29), ECM off. |
Settings: on the machine and in the box
Each brand names its menus its own way; use these values as a starting point.
- Modem speed
- Cap transmission at 9,600 bit/s (V.29). Starting at 14,400 bit/s multiplies failures on IP.
- Super G3 mode (V.34)
- Turn it off: 33,600 bit/s assumes a near-flawless line.
- ECM
- Turned off for G.711. When T.38 behaves reliably, re-enable it and run both modes side by side.
- The router's phone port
- Use the analog port (Tel, Phone, or Fon) connected directly — no splitter, no old ADSL filter. If the interface offers a fax mode, enable it.
- Codec and voice options
- Where the router or ATA allows it: put G.711 A-law first, and turn off silence detection, comfort noise, and echo cancellation.
- Jitter buffer
- Prefer a fixed 60–100 ms buffer over an adaptive one, which recalibrates mid-call.
Which hardware: ATA, gateway, or fax server?
In Belgium, landlines almost always go through the operator's router — Proximus, on copper as on fibre; VOO and Telenet, on cable. Each one exposes an analog phone port, but each handles the fax call in its own way: G.711 passthrough, T.38 relay, or locked settings — it all depends on the box and its firmware. Look in your router's interface, ask the operator, and only then buy hardware.
An ATA converts an analog line into a SIP account: useful if your company uses a VoIP provider or IP PBX. Pick a model that supports T.38, and confirm the SIP trunk permits it as well; if not, expect G.711 fallback.
A multi-port (FXS) gateway powers several fax machines at one site. A fax server, for its part, replaces the machines: incoming faxes as PDFs in email inboxes, sending from workstations — provided you have a T.38 trunk and a dedicated administrator.
Test before concluding it works
Start by plugging a phone into the same port: is there a dial tone? Can you call a landline? If not, it's the line at fault, not the machine.
Next, transmit five to ten pages that combine fine text with dark solid blocks; ask the recipient to report exactly what arrived. Do this at three separate times, including an evening run when the network is busiest, and send one batch the other way to check incoming reception.
Keep the transmission reports: a machine that consistently drops to 4,800 bit/s indicates a fragile line, even when pages get through. Failures persist? Ask your operator, ticket in hand, whether your line supports T.38.
Time to switch to online fax
Keeping a machine behind the router makes sense if faxes regularly arrive on a known number. If you only send, a few sends a month, setup and maintenance quickly cost more than the device.
An online service removes the entire IP chain from your side: the PDF loads in the browser and the service dials the other party's fax machine number. That's how Faxeroo works, with no account creation, to landlines in seven countries, Belgium included. No number is provided, no fax is received: it's a supplement to your receiving line, nothing more.
Fax over IP: frequently asked questions
- Will a fibre connection accept my fax machine?
- Yes: the fibre isn't at fault; the router or ATA determines the outcome. If G.711 passthrough is clean, or T.38 is in use, and the machine is limited to 9,600 bit/s, most sends succeed.
- Should ECM be turned off on a T.38 line?
- Not necessarily. ECM mainly causes trouble when transport is raw G.711. On a line that genuinely negotiates T.38, keep ECM enabled, send a set of test pages, and compare the results against a separate set sent with ECM off.
- Does my router support T.38?
- Browse the router's telephony menus looking for a "fax" or "T.38" option. Otherwise, go through operator support: the answer varies by model and firmware.
Telephony and networks
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- Not a relic, not a snobbery: a tool that solves one precise problem — establishing that a document arrived.
- From Bain's electric clock to the fax sent from a browser: a hundred and eighty years of image transmission.
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